Similar image retrieval, as a type of multimedia recognition technologies, is booming. The similar image retrieval may comprise feature extraction, index construction, inquiries, and similarities sorting. However, conventional technologies of similar image retrieval may present some problems (e.g., poor compatibility and fault tolerance problems) because conventional image signatures do not contain content information of images. For example, two images that have identical content but saved in different formats (e.g., bmp, jpeg, png, or gif) may be considered as different images.